An active pixel is a semiconductor device capable of converting an optical image into an electronic signal. Active pixels can be arranged in a matrix and utilized to generate video signals for video cameras, still photography, or anywhere incident radiation needs to be quantified. When incident radiation interacts with a photosite, charge carriers are liberated and can be collected for sensing. The number of carriers collected in a photosite represents the amount of incident light impinging on the site in a given time-period.
There are two basic devices with many variants, employed to collect and sense, charge carriers in a photosite. The two basic devices are photodiodes and photogates. Variants of photodiodes include, but are not limited to: Pinned, P-I-N, Metal-Semiconductor, Heterojunction, and Avalanche. Photogate structures include: Charge Couple Devices (CCD), Charge Injection Devices (CID) and their variants that include virtual phase, buried channel and other variations that utilize selective dopants. The selective dopants are used to control charge collection and transfer underneath and between the photogate(s) and the sense node.
The solid state imagers heretofore used have been dominated by CCD's because of their low noise as compared to Photodiodes and CIDs. The low noise advantage of CCD imagers is the result of collecting the photon generated charge at the pixel site and then coupling or shifting the actual charge to an amplifier at the periphery of the array. This eliminates the need for the long polysilicon and metal busses that degrade the signal with their associated resistance and capacitance. However, the low noise of the CCD requires the imager to be read in a fixed format and once the charge is read it is destroyed. The requirement of coupling the collected photon charge from the pixel to the periphery amplifier (a.k.a. CTE), requires proprietary processing steps not compatible with industry standards CMOS or BiCMOS processes.
Solid state imaging devices have developed in parallel with CMOS technology and as a result all imager manufacturers developed their own proprietary processes to maximize imager performance characteristics and wafer yield. Specialized silicon wafer processing kept imager prices relatively high. Linear active pixel sensors have been commercially produced since 1985. Beginning in the early 90's the move to transfer the proprietary processes to an industry standard CMOS processes was on. The advantages of using an industry standard process include: competitive wafer processing pricing, and the ability to provide on chip timing, control and processing electronics. By the end of the year 1992 a 512.times.512 CMOS compatible, CID imager with a preamplifier and CDS per column had been fabricated. The imager could either be operated as a random access 512.times.512 CID, or all the columns could be summed together and operated as a linear active pixel sensor.
Area arrays utilizing active pixel sensors in which a photodiode or photogate is coupled to an output source follower amplifier which in turn drives a Correlated Double Sampling (CDS) circuit, where the two outputs of the CDS cell then drives two more source followers circuits that in turn are fed into a differential amplifier are shown in U.S. Pat. No. 5,471,515. This uses source follower circuits, that typically have gains less than unity that vary from one source follower to another. The source follower gain variation is due to variations of FET thresholds. The source follower gain variation results in a pixel to pixel gain mismatch. Also, the active pixel sensors suffer gain variations due to the CDS circuit per column, when the CDS employs a source follower pair to drive its output. The resulting CDS signal and its corresponding offset can have different gains that are not correctable by the differential amplifier. Also, the source follower configuration of active pixels doesn't allow for binning of pixels.
The voltage mode of operation of prior art does not allow for binning, which, is the summation to two or more pixel signals at once.
What is needed is an imager device which has the low noise level of a CCD, the random access, and binning of a CID, and uniform gain and response from all pixels.